1. Field of the Invention
The present invention relates to a heat-conductive composite material having excellent weldability to materials which are attached thereto and excellent surface property of itself, for use, for example, in manufacture of a heat-release substrate (heat sink) for mounting semiconductor chips thereon or of a lead frame in semiconductor packages, the thermal expansion coefficient and the thermal conductivity of the composite material being freely variable in order to satisfy both the compatibility with the thermal expansion coefficient of the materials which are attached thereto, such as metals, ceramics, Si and other semiconductors, and plastics, and the good thermal conductivity of itself so that the heat to be generated by semiconductor chips may efficiently be released out to the outside. In particular, it relates to a heat-conductive composite material which is produced in such a way that a two-layer material or three-layer material to be prepared by laminating a metal foil of high thermal expansion under pressure onto one surface or both surfaces of a metal sheet of low thermal expansion followed by integrating them and thereafter providing a number of through-holes through the integrated body in the thickness direction thereof is laminated under pressure onto one surface or both surfaces of a metal sheet having a high thermal expansion coefficient, which is at room temperature or has been heated to a temperature not lower than the recrystallizing temperature thereof, or in such a way that two-layer materials each to be prepared by laminating under pressure a metal sheet of high thermal expansion, which is at a room temperature or has been heated to a temperature not lower than the recrystallizing temperature thereof, onto one surface of a metal sheet of low thermal expansion having a number of through-holes as provided in the direction of the thickness thereof, or three-layer materials each to be prepared by further laminating under pressure a metal sheet of high thermal expansion onto the other surface of the metal sheet of low thermal expansion of the two-layer material are laminated to each other under pressure directly or optionally via a metal sheet of high thermal expansion. The heat-conductive composite material of the present invention has a high lamination strength, and the thermal expansion coefficient and the thermal conductivity of the material are variable without varying the ratio of the thickness of the selected metal sheets and the ratio of the area of the through-holes. Accordingly, the capacity of receiving heat of the material is uniform and the heat-diffusibility of the material is improved. The surface of the material is free from fine pores so that any thin film may well be coated thereon by plating or brazing. The present invention also relates to a method of producing the heat-conductive composite material.
2. Description of the Prior Art
Recently, integrated circuit chips (hereinafter referred to a "chips") of semiconductor packages, especially those in LSI or ULSI for large-scaled (super) computers, have been directed toward elevation of the integrated degree and acceleration of the operation speed, and therefore the quantity of heat generated by the operation of the devices has become extremely large due to increase of the electric power consumed.
That is, the capacity of the chips has become large-scaled and the quantity of heat generated during the operation has also become large. Accordingly, if the thermal expansion coefficient of the substrate material is significantly different from that of the material of the chip of silicon or gallium arsenide, the chip will peel off from the substrate or be broken.
For this reason, heat-diffusibility has to be taken into consideration in planning the semiconductor package, and therefore the substrate to which the chip is mounted must also have a proper heat-diffusibility. Accordingly, the substrate material is required to have a thermal expansion coefficient near to that of the chip and have a large thermal conductivity.
Conventional semiconductor packages of various constitutions have been proposed. For instance, one constitution is mentioned, in which the substrate is provided with heat-release fins. Composite materials for heat-release substrates (heat sinks), such as clad boards or Cu--Mo or Cu--W alloys for ensuring the heat-releasability, have been proposed (Japanese Patent Application Laid-Open Nos. 59-141247 and 62-294147).
The proposed composite materials meet the satisfactory conditions for practical use with respect to both the thermal expansion coefficient and the thermal conductivity. However, since Mo and W both have a high density and therefore are heavy and brittle, they must be shaped and machined by non-plastic machining such as grinding so as to obtain a shaped body having a determined dimension. Such machining needs a high machining cost so that the yield of the products from the materials is low.
In resin sealed semiconductor packages, not only the lead frame is to be a path for electrical connection with the outside parts but also it has an important role as a heat-releasing path for releasing the heat to be generated in the chips therein. For the reason, a lead frame made of a copper alloy is much used.
However, in the field of needing high reliability, copper alloys involve a danger of peeling the welded interface between a chip and an island, since they have a low mechanical strength and the thermal expansion coefficient of them is not compatible with that of chips. Therefore, in view of the compatibility of the thermal expansion coefficient of the lead frame material with that of a chip, a semiconductor package having an Ni--Fe alloy of low thermal expansion, such as 42% Ni--Fe alloy, has been proposed.
However, since Ni--Fe alloys have a poor thermal conductivity, they could not yield a thermal releasability capable of satisfying the current requirement. In addition, since the difference in the thermal expansion between a chip and a sealant resin is extremely large, the compatibility between a lead frame and the sealant resin is bad so that complete prevention of cracks in the resin is difficult even though the compatibility of the thermal expansion coefficient between the lead frame and the chip is good.
Further, in ceramic semiconductor packages, since sealing is effected by Al wire bonding or glass sealing, an Ni--Fe alloy provided with Al at the bonding area and the sealing position is much used as the lead frame. However, such an Ni--Fe alloy has a poor heat-releasability, as mentioned above, it involves a problem about the compatibility of the thermal expansion coefficient thereof with that of ceramics.
Under the situation, the present applicant already proposed a heat-conductive composite material, so as to solve the above-mentioned problem about the thermal expansion coefficient and/or the thermal conductivity, in which a metal foil of high thermal expansion has been laminated under pressure to the both surfaces of a core material comprising a metal sheet of low thermal expansion having necessary through-holes in the direction of the thickness, as integrated with a metal sheet of high thermal expansion in such a way that the metal of high thermal expansion is exposed out to the surface of the metal sheet of low thermal expansion through the said through-holes, the proportion of the thickness of these metal sheets and the proportion of the area of the through-holes having been selected suitably (Japanese Patent Application No. 2-40550). The proposed composite material is characterized in that the thermal expansion coefficient and the thermal conductivity is variable, that the capacity thereof of receiving heat is made even, that the heat-diffusing effect thereof is improved, that the surface thereof is free from fine pores and that the adhesiveness thereof to a thin film of a plating material or a brazing material is good.
In order to obtain the proposed heat-conductive composite material, a metal sheet of low thermal expansion such as a Kovar sheet is first stamped by pressing to form a number of small through-holes to make it reticulate, then annealed and coiled to make a metal sheet coil of low thermal expansion, the coil is uncoiled with laminating onto the both surfaces thereof a metal sheet coil of high thermal expansion as being uncoiled from both the above and the below of the former coil, and the laminated sheets are rolled and welded under pressure with large-diameter rolls by cold rolling or warm rolling and annealed for diffusion to obtain a core material, then a metal foil of high thermal expansion, such as Cu or Al foil, is laminated onto the both surfaces of the core material with the coil of the metal foil being uncoiled from both the above and the below of the core material, and the laminated core material is rolled and welded under pressure with rolls by cold rolling or warm rolling and annealed for diffusion.
In manufacturing the heat-conductive composite material, a metal foil of high thermal expansion such as Cu or Al foil is laminated onto the both surfaces of the core material and welded by cold rolling. In the step, if the reduction ratio is made too large so as to obtain a high welding strength, the circular or oval shape of the exposed area of the metal of high thermal expansion such as Cu or Al as exposed onto the surfaces of the core material would be a long oval shape whereby the selected proportion of the surface area between the metal of high thermal expansion and the metal of low thermal expansion would vary and accordingly the thermal expansion coefficient and/or the thermal conductivity of the laminated material would vary or the thermal expansion coefficient of the same would be anisotropic, defectively.
In order to evade the defective problem, it may be considered to employ a plating method in place of the rolling method, for coating the metal foil of high thermal expansion such as Cu or Al foil over the surfaces of the core material. According to the alternative method, however, a plating solution would remain in the interface between the metal of high thermal expansion and the metal of low thermal expansion on the surfaces of the core material after the material has been dipped in a plating bath, and this would be gasified in the subsequent diffusion-annealing step to often cause swelling or peeling of the plated layer. In addition, the plating step is generally complicated and needs much time, unfavorably.